Substrates of this type are used as interposers for electrically connecting the terminals of a plurality of homogeneous or heterogeneous microchips. The interposer generally consists of glass or silicon and contains, for example, contact faces, rewiring connections, through-plating and active and inactive components.
As a processor core, a microchip typically has several hundred contact points, narrowly spaced apart from one another, distributed over a relatively small area on the lower face thereof. Because of this narrow spacing, these contact points cannot be applied directly to a circuit board, known as the motherboard. An interposer, by means of which the contacting base can be spread, is therefore used as a connecting element.
In practice, a glass-fibre-reinforced epoxy resin plate, for example, is used as an interposer and is provided with a number of holes. Conductor traces extend on the upper face of the glass fibre mat, and lead into the respective holes so as to fill them, and lead to the terminal contacts of the processor core on the other face of the glass fibre mat. In the event of heating, however, different expansions occur in the core processor and in the glass fibre mat, resulting in mechanical stresses between these two components.
Therefore, to reduce the stresses resulting from the different thermal expansion coefficients, silicon interposers are also used. The silicon interposers can be processed in the manner conventional in the semiconductor sector. However, silicon-based interposers are very expensive to produce, and so efforts are increasingly being made to replace them with more cost-effective glass material, since the thermal expansion of glass can be matched to that of silicon.
In this context, it is found to be a challenge to process the glass into usable interposers. The prior art has not yet addressed, in particular, the economical production of the plurality of through-openings in the substrate for through-plating.
EP 2 503 859 A1 thus discloses a method in which a substrate is provided with through-holes, the substrate consisting of an insulator such as glass, for example silicate glass, sapphire, plastics material or ceramic and semiconductors such as silicon. The substrate is irradiated using a laser, for example a femtosecond laser, which is focused on a focal point at a desired position within the substrate. The through-holes are produced by a method in which the regions of the substrate which have been modified by the laser are dipped in an etching solution and the modified regions are thus removed from the substrate. This etching makes use of the effect whereby the modified region is etched extremely rapidly by comparison with the unmodified regions of the substrate. Blind holes or through-openings can be produced in this manner. A copper solution is suitable for filling the through-opening. To achieve a desired depth effect, in other words a through-hole between the outer substrate faces, the focal point has to be displaced during continuous irradiation, in other words tracked in the direction of the z-axis.
More generally, the combination of selective laser treatment with a subsequent etching process in the form of selective laser-induced etching is also known as ISLE (in-volume selective laser-induced etching). This method is used to produce microcomponents, tracks and shaped details in transparent materials such as glasses or sapphire. The miniaturisation of products for microoptics, medical technology and microsystem technology requires the production of components having dimensions in the micrometre range and having structural precisions of up to 100 nm. The ISLE method is a suitable production method for structures made of and made in transparent materials. As a result of the laser radiation being focused in the interior of the workpiece, the material is structurally altered in a small volume (a few cubic micrometres). For example, the crystalline structure of sapphire is converted to an amorphous vitreous structure, which is etched 10,000 times more rapidly than the starting material. By moving the laser focus through the workpiece, coherent modified areas are produced, which are subsequently chemically etched in aqueous solution using potassium hydroxide or hydrofluoric acid and removed.
DE 10 2010 025 966 B4 discloses a method in which in a first step focused laser pulses are directed onto the substrate, the radiation intensity of said pulses being high enough to result in local athermal decomposition along a filament-like track in the glass. In a second method step, the filament-like tracks are expanded into holes by supplying high-voltage power to opposing electrodes, resulting in dielectric breakdowns through the substrate along the filament-like tracks. These breakdowns expand under electrothermal heating and evaporation of hole material, until the process is halted by switching off the power supply upon achieving the desired hole diameter. Alternatively or in addition, the tracks may also be expanded using reactive gases, which are directed onto the hole sites using nozzles. The through-opening sites may also be expanded using supplied etching gas. The comparatively complex process, resulting from the fact the substrate initially has to be broken through by the athermal decomposition and the diameter of the filament-like tracks has to be expanded into holes in the following step, has proved to be disadvantageous.
Further, U.S. Pat. No. 6,400,172 B1 discloses the introduction of through-openings in semiconductor materials by laser.